(1) Field of the Invention
The present invention relates to semi-continuous vacuum heat-treating furnace, and more particularly to a semi-continuous vacuum heat-treating furnace which is composed of two chambers, a vacuum heating chamber and a cooling chamber, and an intermediate vacuum door to separate both chambers from each other, and its operation process.
(2) Description of Prior Art
Batch type heat-treating furnaces of hot wall style and cold wall style have been conventionally utilized for a vacuum heat-treating furnace. However, these furnaces have disadvantages in
(1) that if heated substance surface, heating element and heat insulation material are not cooled to a temperature at which oxidation does not occur, heated substance cannot be discharged out of the furnace and subsequent substance to be heated cannot be charged into the furnace, and PA0 (2) that most of heating energy required to heat substance to a prescribed temperature is dissipated to raise temperature of heat insulation substance and a furnace floor.
In order to eliminate above mentioned disadvantages, a semi-continuous vacuum heat-treating furnace of three-chambers and two-doors type was developed. As shown in FIG. 1, the heat-treating furnace comprises a heating chamber B including a heating element B1 and a heat insulation material B2 and for heating a substance M at vacuum condition or low-pressure condition, a preparation chamber A disposed at forward side of the heating chamber B, a cooling chamber C having a cooling fan E and disposed at rear side of the heating chamber B, first and second intermediate vacuum doors G, H for separating above mentioned three chambers A, B, C from each other, a charging door F and a carriage door I.
The substance M heated to a high temperature in the heating chamber B is transferred through the second intermediate vacuum door H opened to the vacuum cooling chamber C. After closing the door H, the cooling chamber C is evacuated into low-pressure condition and then the substance M is cooled by means oil quenching or non-oxidizing gas such as nitrogen or argon. After the preparation chamber A is evacuated into vacuum condition, the first intermediate door G is opened and successive substance M is transferred to the vacuum heating chamber B at high temperature. In this constitution, waiting time for heating the substance M is significantly reduced and the heating chamber B can be always held in vacuum condition of high temperature. Accordingly, this constitution is superior to conventional batch type in working ratio, thermal efficiency, productivity and energy saving.
Operation process of above mentioned semi-continuous vacuum heat-treating furnace particularly non-oxidizing gas cooling furnace will now be described referring to Table 1.
The vacuum heating chamber B is previously heated to a prescribed temperature at vacuum condition, the preparation chamber A and the cooling chamber C are held in atmospheric pressure, and the charging door F, the first intermediate vacuum door G, the second intermediate vacuum door H and the carriage door I are all closed.
At the first step, the charging door F is opened, the first substance M1 is entered to the preparation chamber A in atmospheric pressure, and then the charging door F is closed.
At the second step, after evacuating the preparation chamber A into vacuum condition, the first intermediate vacuum door G is opened, the first substance M1 is transferred from the preparation chamber A to the vacuum heating chamber B, and the first intermediate vacuum door G is immediately closed.
At the third step, the first substance M1 is heated to a prescribed temperature in the vacuum heating chamber B and held at the temperature for a prescribed time. The preparation chamber A is restored to atmospheric pressure condition, the charging door F is opened, second substance M2 is entered to the preparation chamber A, and the charging door F is immediately closed and the cooling chamber C is evacuated into low-pressure condition.
At the fourth step, the preparation chamber A is evacuated into vacuum condition, the cooling chamber C at low-pressure condition is made vacuum condition, the second intermediate door H is opened, the first substance M1 at high temperature is transferred from the vacuum heating chamber B to the cooling chamber C, and the second intermediate door H is immediately closed.
At the fifth step, the first intermediate vacuum door G is opened, the second substance M2 is transferred from the preparation chamber A to the vacuum heating chamber B, cooling gas is introduced to the cooling chamber C and stirred by the cooling fan E while the cooling chamber C is restored to low-pressure condition, the first substance M1 is cooled at a prescribed cooling rate until a prescribed temperature is attained, and then the cooling chamber C is restored to atmospheric pressure.
At the sixth step, the carriage door I is opened and the first substance M1 in the cooling chamber C is discharged out of the furnace. At the same time, the second substance M2 is heated to a prescribed temperature in the vacuum heating chamber B and held at the temperature for a prescribed time. On the other hand, the preparation chamber A is restored to atmospheric pressure, the charging door F is opened, the third substance M3 is entered, and then the charging door F is immediately closed.
At the seventh step, operation process is performed in similar manner to the fourth step. Process is repeated in the fourth, first and sixth steps in sequence at normal operation condition.
The semi-continuous vacuum heat-treating furnace of three-chambers and two-doors type which repeatedly performs above mentioned process can obtain above mentioned technical effect. However, the heat-treating furnace of this type inevitably becomes large in length therefore requires large-scale devices in evacuating system and complicated devices in control system and heated substance transferring system. Accordingly, such furnace has disadvantages in high manufacturing cost, long maintenance time and long waiting time for heating the substance.
Soot (carbon powder) produced at vacuum carburizing in this heat-treating furnace cannot be removed by burning and it is gradually deposited to the heating element in the heating chamber. The deposit of soot reduces electric resistance of the heating element and may cause short-circuit fault at the worst case, resulting in stop of operation. Soot adhering to heat insulation substance decreases insulating effect thereof and electric power required for heating increases. In addition, constitution of the heating chamber disposed between the preparation chamber and the cooling chamber makes maintenance of the heating chamber difficult.